Based on local unitary operators acting on n-dimensional Hilbert spaces, we investigate selective and collective operator basis sets for N-particle quantum networks. Selective cluster operators are used to derive the properties of maximum entangled states (Bell states) for any n and N. Collective operators are convenient to account for fundamental or operational indistinguishability. For permutation symmetry the respective Hilbert-space dimension is only polynomial in N, and is governed by strong selection rules. These selection rules can be exploited for the design of decoherence-free subspaces as well as for the implementation of efficient routes to entanglement if suspended switching between states of different symmetry classes could be realized.
The analysis of an open quantum system can be by far difficult if the dimension of the system Hilbert space is large or infinite. However, in some cases the dynamics on a finite-dimensional Hilbert space can be decomposed into a block-diagonal form, which simplifies the system structure. In this presentation, we will study several criteria for the complete reducibility and, in addition, a computational method for a basis of each simplified component to apply for the analysis of open quantum systems. An important point of these tools is that they are “effective” methods (one can complete the task in a finite number of steps).